So, Pythgoras Theorem mostly deals with right-angled triangles, ie a triangle with one 90 degree angle.
The slanted side is called the hypotenuse and let us denote it as c the bottom is called the adjacent, let's denote it a and the standing side is called opposite, let's denote it b.
Now, Pythgoras Theorem states that:
c^2 = a^2 + b^2
This theorem is often used to find the sides of triangles which can then be used to find other angles in the triangles.
And there you have it, Pythgoras Theorem!